In recent years, instead of using aluminum or aluminum alloys as a material for forming interconnection circuits on a substrate such as a semiconductor wafer, there is a strong movement towards using copper (Cu) which has low electric resistance and high electromigration resistance. Copper interconnects are generally formed by filling copper into fine recesses formed in a surface of a substrate. There are known various techniques for forming such copper interconnects, including CVD, sputtering, and plating. According to any such technique, a copper film is formed on substantially an entire surface of a substrate, followed by removal of unnecessary copper by chemical mechanical polishing (CMP).
FIGS. 22(a) through 22(c) illustrate, in sequence of process steps, an example of forming such a substrate W having copper interconnects. As shown in FIG. 22(a), an insulating film 2, such as a silicon oxide film/a film of silicon oxide (SiO2) or a film of low-k material, is deposited on a conductive layer 1a in which electronic apparatus are formed, which is formed on a semiconductor base 1. A contact hole 3 and a trench 4 for interconnects are formed in the insulating film 2 by performing lithography and an etching technique. Thereafter, a barrier layer 5 of TaN or the like is formed on an entire surface, and a seed layer 7 as an electric supply layer for electroplating is formed on the barrier layer 5.
Then, as shown in FIG. 22(b), copper plating is provided on a surface of substrate W to fill the contact hole 3 and the trench 4 with copper and, at the same time, deposit a copper film 6 on the insulating film 2. Thereafter, the copper film 6 on the insulating film 2 is removed by chemical mechanical polishing (CMP) so as to make a surface of the copper film 6, filled in the contact hole 3 and the trench 4 for interconnects, and a surface of the insulating film 2 lie substantially in the same plane. An interconnection composed of the copper film 6 as shown in FIG. 22(c) is thus formed.
Components in various types of equipment have recently become smaller, thereby requiring a high degree of accuracy. As sub-micro manufacturing technology has commonly been used, properties of materials are largely influenced by a processing method. Under these circumstances, in such a conventional processing method that a desired portion in a workpiece is physically destroyed and removed from a surface thereof by a tool, a large number of defects may be produced to deteriorate properties of the workpiece. It is important therefor to be able to perform processing without deteriorating properties of materials. Some processing methods, such as chemical polishing, electrolytic processing, and electrolytic polishing, have been developed in order to solve this problem. In contrast with conventional physical processing, these methods perform removal processing or the like through a chemical dissolution reaction. Therefore, they do not suffer from defects, such as formation of an altered layer and dislocation, due to plastic deformation, whereby processing can be performed without deteriorating properties of materials.
On the other hand, an electrolytic processing method and/or apparatus using an ion exchanger has been developed. In this method, as shown in FIG. 23, after an ion exchanger 512a mounted on a processing electrode 514 and an ion exchanger 512b mounted on a feeding electrode 516 are brought into contact with or close to a surface of a workpiece 510, liquid 518, e.g. ultrapure water, is supplied from a liquid supply section 519 between the processing electrode 514 and the feeding electrode 516, and the workpiece 510, while a voltage is applied from a power source 517 between the processing electrode 514 and the feeding electrode 16 to thereby perform a removing process of a surface of the workpiece. According to this electrolytic processing, water molecules 520 in the liquid 518 such as ultrapure water are dissociated by the ion exchangers 512a, 512b into hydroxide ions 522 and hydrogen ions 524. The hydroxide ions 522 thus produced, for example, are carried, by an electric field between the workpiece 510 and the processing electrode 514 and by flow of the liquid 518, to a surface of the workpiece 510 opposite to the processing electrode 514, whereby a density of the hydroxide ions 522 in the vicinity of the workpiece 510 is enhanced, and the hydroxide ions 522 are reacted with atoms 510a of the workpiece 510. A reaction product 526 produced by this reaction is dissolved in the liquid 518, and removed from the workpiece 510 by flow of the liquid 518 along a surface of the workpiece 510. A removing process of the surface of the workpiece 510 is thus effected.
As explained above, if an electrolytic process is performed by disposing an ion exchanger adjacent to at least one of a processing and feeding electrode and a workpiece, control at an end of processing becomes difficult.
Namely, when electrolytic processing is performed in a state where a current flowing between a processing electrode and a feeding electrode is controlled at a constant level, as a principle, a processing rate is kept constant unless an area to be processed changes, and because of this feature control during processing becomes easier, and in addition a totalized current value can be calculated easily, so that an amount of processing and a processing end point can easily be grasped. In association with progress of polishing, however, when barrier layer 5 comprising an insulating body (See FIG. 22(a)) is exposed on a surface of wafer W upon completion of electrolytic processing, an area to be processed decreases depending on a line/space ratio as well as on the wiring density, which may cause a rapid increase in a processing rate.
Further when a conductive film such as copper coating 6 (See FIG. 22(a) or FIG. 22(c)) as a material to be processed on a surface of the wafer W is removed, an electric resistance value of the conductive film becomes larger as a film thickness becomes smaller, and therefore when electrolytic processing is performed keeping a current at a constant level, a loaded voltage increases in association with reduction of film thickness, and an increasing rate becomes higher as a processing point comes closer to a processing end point where a wiring pattern is exposed on a surface of the wafer. This phenomenon occurs because applied voltage is inversely proportional to film thickness, and when voltage rapidly increases as described above, control over the processing end point is difficult. In addition, when the applied voltage increases over a predetermined value, dielectric breakdown (a so-called electric discharge) occurs in ultrapure water, which may cause physical damage to a workpiece.
On the other hand, when electrolytic processing is performed keeping a voltage applied to between a processing electrode and a feeding electrode at a constant level, a processing rate rapidly drops in association with rapid reduction of an area to be processed. Namely, in association with progress of polishing, when the barrier layer 5 comprising an insulating body (See FIG. 22(a) or FIG. 22(c)) is exposed on a surface of wafer W upon completion of processing, the area to be processed decreases, which makes it difficult for an electric current to flow therethrough, so that the processing rate rapidly drops, and thus the processing rate varies, so that it becomes difficult to detect a processing end point.
The processing end point means, as used herein, a point of time when processing has been performed up to a predetermined amount of processing for a specified section of an area to be processed, or for any parameter correlating to an integrated processing rate. As described above, by making it possible to freely set a processing end point even during processing, electrolytic processing in a multistage process is enabled.
Further, as described above, when it is attempted to remove copper used for coating substantially an entire surface of a substrate only by chemical-mechanical polishing (CMP), since a polishing liquid is generally used in the chemical-mechanical polishing, not only is it required to fully clean a semiconductor substrate contaminated by the polishing liquid after an end of polishing, but also there occur such problems as cost for the polishing liquid itself as well as for chemicals required for cleaning, and negative influences caused by this processing over an environment. Therefore, there is a strong need for alleviating disadvantages of CMP.
Although a process of polishing a wafer by CMP while plating is being performed has been reported, when mechanical processing is applied to a plating growth surface, sometimes abnormal growth of plating may be promoted, which may in turn cause abnormality in film quality. It has also been reported that, in electrolytic processing or in electrolytic polishing described above, processing proceeds in association with progress of an electro-chemical mutual reaction between a workpiece to be processed and an electrolytic solution (an aqueous solution of NaCl, NaNO3, HF, HCl, HNO3, NaOH or the like). Therefore, when the electrolytic solution containing an electrolyte as described above is used, a workpiece to be processed will inevitably be contaminated.
Further, an electrolytic processing method using an ion exchanger and deionized water, and preferably ultrapure water has been developed. Generally a plated substrate has fine irregularities on a surface (plated surface), and in this electrolytic processing method, deionized water is present also in concave sections of a substrate's surface, and as the deionized water itself is ionized little, a process for removing unnecessary materials from the substrate barely proceeds in sections contacting the deionized water in the concave sections. Therefore, a process for removing unnecessary materials proceeds only in sections contacting the ion exchanger containing abundant ions therein, and this method is advantageously more excellent in its capability for flattening a surface of a substrate as compared to a conventional electrolytic processing method using an electrolytic solution.
If an ion exchanger with low elasticity, namely a soft and easily-deformable ion exchanger is used, the ion exchanger follows irregularities on a surface of the substrate, and it is difficult to eliminate the irregularities on the substrate's surface by selectively processing convex sections thereon.